This invention relates to a molten matter discharging apparatus for efficiently and reliably discharging molten matter formed inside a furnace when substances to be melted such as metals are induction-heated by using a cold crucible induction melting furnace.
A cold crucible induction melting furnace has a construction in which a divided, water-cooled metallic cold crucible is disposed inside a water-cooled high-frequency coil. When substances to be melted such as metals are charged into such melting furnace and a high-frequency current is supplied to the high-frequency coil, the metals are induction-heated and are converted to molten matter. In this instance, a floating force acts on the molten matter itself due to an electromagnetic field, and the molten matter does not come into direct contact with a furnace body of the melting furnace. Therefore, such induction melting furnace has the capability that materials having high melting points can be melted and erosion of the furnace body by the molten matter scarcely occurs. Furthermore, because the furnace body itself is cooled with water, high temperature melting of the substances to be melted can be achieved without being limited by the heat-resistant temperature of the furnace body. For these reasons, the cold crucible induction melting furnace has been utilized at present for melting special metals in the iron and steel industry.
On the other hand, a method for collectively and conveniently melting radioactive miscellaneous solid wastes including a variety of substances such as combustibles, metals, glass and other non-combustibles generated from nuclear facilities and the like by using such a cold crucible induction melting furnace has been proposed. (See U.S. Pat. No. 5,457,264 corresponding to Japanese Patent Laid-open No. 7-63895/1995; hereinafter referred to as "prior art method".)
In this prior art method, when the radioactive miscellaneous solid wastes are charged into the cold crucible induction melting furnace and a high-frequency current is supplied to the high-frequency coil, conductive substances contained in the miscellaneous solid wastes, such as metals, are first induction-heated and are melted. Due to the heat generated at this time, the remaining miscellaneous solid wastes having a low conductivity surrounding the metals also are indirectly heated. In other words, the metals function as a starting source of heating and the miscellaneous solid wastes are entirely melted.
By the prior art method, the molten metal does not come into direct contact with the furnace body because the floating force acts on the molten matter due to the operation of the electromagnetic field as described above. Also in the case of glass melting, the contact surface of the molten glass with the furnace body is cooled and is converted to a solid layer (skull layer), so that the direct contact of the high temperature molten glass with the furnace body does not occur. Thus, high temperature erosion of the furnace body does not occur, and high temperature melting of the substances to be melted becomes possible.
In order to carry out a continuous melting operation by using the cold crucible Induction melting furnace described above, the high temperature molten matter must be discharged from the furnace. Conventional methods of discharging the molten matter include a system which allows the molten matter to overflow from the furnace top by tilting the melting furnace itself, a system which allows the molten matter to flow down from an outflow port at the furnace bottom portion by pressurizing the inside of the furnace, and the like. However, the former system requires a moving structure for tilting the furnace body, and the latter requires a gas-tight structure of the furnace body.
On the other hand, a nozzle heating system (a freeze valve system) has been employed in the past for a glass melting furnace used in vitrification of high-level radioactive wastes. This system has a construction wherein heating means is disposed around a discharging nozzle extending downwardly from the furnace bottom portion. Since the molten glass inside the nozzle is solidified when the discharging nozzle is not heated, the molten glass inside the furnace does not flow down and is not discharged. To discharge the molten glass inside the furnace, the discharging nozzle is heated so as to melt the solidified glass inside the nozzle and allow it to flow down by gravity, and at the same time, the molten glass inside the furnace can be discharged.
As nozzle heating means in such nozzle heating system, there has been proposed a high-frequency heating means wherein a high-frequency coil is disposed around a metallic discharging nozzle and a high-frequency current is supplied to this coil to heat the nozzle. However, when this nozzle heating system is adopted as a molten matter discharging apparatus in a cold crucible induction melting furnace and the nozzle is heated by high-frequency heating, there remain the problems that the metallic furnace body and the metallic discharging nozzle are electrically short-circuited, and noise interference occurs between the high-frequency heating system for heating the furnace body and the high-frequency heating system for heating the discharging nozzle.